This invention relates to a copper base lead material for leads in semiconductor devices, and more particularly to such lead material which possesses sufficient strength which is particularly required of a metal material for leads in semiconductor devices having high wiring densities, while also exhibiting excellent heat radiation or thermal conductivity.
A semiconductor device such as an integrated circuit (hereinafter called "IC") is generally manufactured, for instance, by the following main processes (a)-(f):
(a) preparing a strip for leads having a thickness within a range from 0.1 to 0.3 mm as a metal material for leads;
(b) stamping the strip for leads into a lead frame having a shape suitable for that of the semiconductor device which is to be manufactured;
(c) mounting IC chips on the lead frame at its predetermined surface portions, either by hot bonding using a conductive resin such as Ag paste, or by thermocompression bonding through one or more plating layers of at least one selected from the group consisting of Au, Ag, and Ni which has been previously plated on the surface of the strip or the lead frame;
(d) connecting elements of the IC chips to the lead frame by wire bonding using Au wires or the like;
(e) encapsulating the IC chips, the wire bonding portions, the surface portions of the lead frame on which the IC chips have been mounted, etc. by a plastic resin or the like for protection of same; and
(f) cutting the lead frame along boundaries between adjacent ones of the IC chip-mounted surface portions, to form leads of the semiconductor device.
Therefore, a metal material for leads of a semiconductor device is required to satisfy properties such as excellent stampability, thermal resistance enough to be free of thermal strain as well as thermal softening when IC chips are bonded to a lead frame stamped from the material for leads, excellent heat radiation, i.e. thermal conductivity (since the thermal conductivity is proportional to the electrical conductivity, the heat radiation can be measured in terms of electrical conductivity), and particularly sufficient strength enough to be free of breakage during transportation of the semiconductor device or when the leads are repeatedly bent in connecting them to wiring boards during mounting of the semiconductor device into an electrical device or an electronic device. That is, a metal material for leads of a semiconductor device such as a plastic leaded chip carrier (PLCC) type IC device is required to possess the following satisfactory properties:
Tensile Strength (for evaluation of the strength): not less than 50 Kg/mm.sup.2 ; PA1 Elongation: not less than 5%; PA1 Electrical Conductivity (for evaluation of the heat radiation and electrical conductivity): not less than 80% I.A.C.S. (International Annealed Copper Standard); and PA1 Softening Point (for evaluation of the thermal resistance): not less than 350.degree. C. PA1 Tin: from 0.05 to 0.25 percent by weight; PA1 Silver: from 0.01 to 0.2 percent by weight; PA1 Phosphorus: from 0.25 to 0.1 percent by weight; PA1 Magnesium: from 0.05 to 0.2 percent by weight; and PA1 Copper and inevitable impurities: the balance, wherein the ratio of phosphorus to magnesium (P/Mg) is within a range from 0.5 to 0.85, preferably 0.60 to 0.85; so as to form a compound of magnesium and phosphorus (Mg.sub.3 P.sub.2).
There has been a recent demand for a lead material having a reduced thickness so as to meet a recent demand for more lightweight and higher performance semiconductor devices. Conventionally, copper base alloys of high strength and relatively high electrical conductivity have been proposed, e.g., by British Pat. No. 2,123,032A, which comprises 0.05 to 0.2% tin, 0.05 to 0.25% magnesium, optionally up to 0.4% silver, optionally up to 0.01% phosphorus, and the balance of copper and impurities. However, the alloys of the British patent do not possess strength sufficient to fulfill the recent requirements, particularly reduced thickness.